1. Field of the Invention
The present invention relates to a nitride semiconductor laser element and to a fabrication method thereof. More particularly, the present invention relates to a nitride semiconductor laser element having a nitride semiconductor laid on a substrate, like a nitride semiconductor substrate, that has a defect-concentrated region, and to a fabrication method of such a nitride semiconductor laser element.
2. Description of Related Art
Nitride semiconductors are compounds of a group III element—such as Al, Ga, or In—and N, which is a group V element. For their band structure and chemical stability, nitride semiconductors have been arousing expectations as materials for light-emitting elements and power devices, and have been tried in various applications. Particularly many attempts have been made to fabricate, as light sources for optical information recording apparatuses, nitride semiconductor laser elements that emit blue light.
In such a nitride semiconductor laser element, using a nitride semiconductor substrate that has the same cleavage direction as the nitride semiconductor layer laid on the surface thereof helps improve the lattice matching between the substrate and the nitride semiconductor layer laid thereon, and also helps eliminate a difference in thermal expansion coefficient. In that way, it is possible to reduce the strains, defects, and the like that develop in the nitride semiconductor laser element, and thereby to extend its lifetime. Inconveniently, however, a nitride semiconductor substrate contains defects (such as voids, interstitial atoms, and dislocations, which disturb the regularity of the crystal), and the density of such defects strongly affects the lifetime of the nitride semiconductor laser element.
Thus, in nitride semiconductor substrates, reduced defect densities are sought. One publicly reported method for fabricating a GaN substrate with a low defect density is as follows (see Applied Physics Letter. Vol. 73 No. 6 (1998) pp. 832-834). By an MOCVD (metal organic chemical vapor deposition) process, on a sapphire substrate, a 2.0 μm thick GaN layer is grown and then, further thereon, a 0.1 μm thick SiO2 mask pattern having periodic stripe-shaped openings (with a period of 11 μm) is formed; then, again by an MOCVD process, a 20 μm thick GaN is formed. In this way, a wafer is produced.
This is a technology called an ELOG (epitaxial lateral overgrowth) process, which exploits lateral growth to reduce defects. Subsequently, by an HVPE (hydride vapor phase epitaxy) process, a 200 μm thick GaN layer is formed, and then the sapphire substrate that forms the base layer is removed. In this way, a 150 μm thick GaN substrate is produced, and its surface is then polished flat. It is known that the GaN substrate thus produced has a dislocation density as low as 106 cm−2 or less.
Inconveniently, however, even when a nitride semiconductor laser element is fabricated by laying a nitride semiconductor layer on such a low-defect nitride semiconductor substrate, since the nitride semiconductor layer itself is composed of different kinds of film such as GaN, AlGaN, and InGaN, the differences in lattice constant among the individual films forming the nitride semiconductor layer produce lattice mismatch. As a result, the nitride semiconductor layer develops strains and cracks, which greatly influence the deterioration and hence the yield of the nitride semiconductor laser element.
Under this background, there has been developed the following method. A nitride semiconductor substrate is used that has, formed on the surface thereof, a groove, i.e., a lower-leveled portion, and a ridge, i.e., a higher-leveled portion. On this nitride semiconductor substrate, a nitride semiconductor layer is grown. This releases the strains in the nitride semiconductor layer, and thus helps reduce cracks. With this method, it is possible to reduce the cracks and strains attributable to the substrate and also the cracks and strains attributable to the lattice mismatch among the individual films forming the nitride semiconductor layer formed on the substrate. In this way, it is possible to alleviate the deterioration of and hence improve the yield of the nitride semiconductor laser element (see JP-A-2004-356454 and JP-A-2005-064469).
FIG. 11 is a cross-sectional view of the conventional nitride semiconductor laser element just described. This nitride semiconductor laser element 50 has an n-type GaN substrate 501 as a nitride semiconductor substrate. The n-type GaN substrate 501 has, in a part thereof, a defect-concentrated region 518, and has, elsewhere than in the defect-concentrated region 518, a low-defect region. On the n-type GaN substrate 501, a nitride semiconductor layer is grown epitaxially. Thus, this nitride semiconductor layer also has, in a part thereof, a defect-concentrated region 518a grown from the defect-concentrated region 518 of the n-type GaN substrate 501, and have, elsewhere than in the defect-concentrated region 518a, a low-defect region. Moreover, the n-type GaN substrate 501 is etched in a part thereof where it has the defect-concentrated region 518 to form a stripe-shaped groove, so that, in a part where the defect-concentrated regions 518 and 518a of the n-type GaN substrate 501 and the nitride semiconductor layer are located, a groove 500 is formed that appears etched relative to the low-defect region.
More specifically, in the n-type GaN substrate 501, above the defect-concentrated region 518, a 6 μm deep groove 500a is formed. Then, on this n-type GaN substrate 501, a nitride semiconductor layer is laid through semiconductor growth using an MOCVD (metal organic chemical vapor deposition) method. The nitride semiconductor layer thus formed on the n-type GaN substrate 501 is composed of, from the bottom up: a lower contact layer 502 formed of n-type GaN; lower three-layer clad layers 503 formed of n-type AlGaN of different compositions; a lower guide layer 504 formed of n-type GaN; an active layer 505 composed of a multiple quantum well structure formed of InGaN; an evaporation prevention layer 506 formed of p-type AlGaN; an upper guide layer 507 formed of p-type GaN; an upper clad layer 508 formed of p-type GaN; and an upper contact layer 509 formed of p-type GaN.
Furthermore, the nitride semiconductor laser element 50 has a ridge stripe 510 formed on the low-defect region of the nitride semiconductor layer. Moreover, over both side walls of the ridge stripe 510 and over the etched floor surface that appears when the ridge stripe 510 is formed, a burying layer 511, which is a dielectric layer formed of SiO2, is laid to produce a stepwise refractive index distribution parallel to the active layer 505 and thereby to achieve confinement in a horizontal lateral mode. The burying layer 511 also serves as a current constricting layer, and thereby permits electric power to be supplied only via the summit of the ridge stripe 510. To enable the nitride semiconductor laser element 50 to receive electric power from outside, a p-type electrode 512 is deposited over the summit of the ridge stripe 510 and over the burying layer 511, and an n-type electrode 513 is deposited over the entire bottom surface of the nitride semiconductor laser element 50.
The nitride semiconductor laser element 50 structured as described above achieves confinement of light with the stepwise refractive index distribution in the ridge stripe 510, and thereby achieves stable lasing in a horizontal lateral mode. When actually fabricated samples of this nitride semiconductor laser element are operated at 60° C., at a low output of 30 mW, a proportion as large as 80% of them have lifetimes of 3 000 hours or more. Thus, by forming in the wafer a groove that produces a carved region 500 as described above, it is possible to achieve extremely high yields.
Inconveniently, however, when conventional nitride semiconductor laser elements 50 fabricated by common processes such as photolithography, vacuum evaporation, polishing, cleaving, and coating are operated in CW (continuous wave) lasing up to an output as high as 100 mW or more, a certain proportion of them end up in element breakdown without reaching a sufficient optical output to achieve the desired reliability. Among these conventional nitride semiconductor laser elements, the proportion of those that end up in element breakdown increases as the duration they are operated increases. Thus, depending on the operating conditions, it may occur that most of the fabricated nitride semiconductor laser elements 50 do not offer the desired reliability. As a result, when conventional nitride semiconductor laser elements are fabricated as elements to be operated at an output as high as 100 mW or more, not only are their yields extremely low, but also, when actually operated for long durations, they are liable to suffer sudden breakdown.
Investigating the Cause of Failure of Nitride Semiconductor Laser Elements
To investigate the cause of the breakdown that a conventional nitride semiconductor laser element 50 structured as shown in FIG. 11 suffers before reaching a sufficient optical output, we, the applicant of the present application, conducted an examination of the nitride semiconductor laser element 50. Specifically, to investigate the cause of the breakdown of the nitride semiconductor laser element 50, with samples thereof that suffered breakdown, we removed the coating laid on a mirror facet thereof, and examined a ridge stripe 510 portion of the mirror facet under an electronic microscope.
Through the examination of the ridge stripe 510 portion of the mirror facet under an electronic microscope, we confirmed that, as shown in FIG. 12, which is an enlarged schematic view of the ridge stripe 510 portion, a surface irregularity 517 had developed on the mirror facet of the nitride semiconductor layer, the surface irregularity 517 extending parallel to the nitride semiconductor layer. This surface irregularity 517 that developed on the mirror facet had a shape as shown in FIG. 13A or 13B, either of which is an enlarged schematic cross-sectional view taken along line A-A shown in FIG. 12.
Here, the side where the p-type electrode 512 was located was the top side, and, whereas the cleavage facet 520 of the part of the nitride semiconductor layer located above the surface irregularity 517 and the cleavage facet 521 of the part of the nitride semiconductor layer located below the surface irregularity 517 were both flat, with a surface roughness of about 3 Å in RMS value, the surface irregularity 517 was as large as several tens of nanometers. That is, either, as shown in FIG. 13A, the cleavage facet 521 below the surface irregularity 517 projected relative to the cleavage facet 520 above the surface irregularity 517 or, as shown in FIG. 13B, the cleavage facet 520 above the surface irregularity 517 projected relative to the cleavage surface 521 below the surface irregularity 517. Moreover, as shown in FIG. 14, which is a schematic view of the entire cleavage facet, the surface irregularity 517 extended parallel to the nitride semiconductor layer over a length of several tens of micrometers to several hundred micrometers.
We then examined particularly closely, of the above surface irregularity 517 that developed on the mirror facet of the nitride semiconductor laser element 50, the portion where the ridge stripe 510 confined light. We thereby found that the surface irregularity 517 concentrated at the interfaces between the individual layers, such as the active layer 505 and the evaporation prevention layer 506, located between the lower guide layer 504 and the upper guide layer 507. On the other hand, we also confirmed that, without such a surface irregularity 517, the nitride semiconductor laser element 50 offered sufficient reliability when operated at an output as high as 100 mW or more.
As described above, conventional nitride semiconductor laser elements are prone to develop a surface irregularity on the mirror facet, the surface irregularity extending parallel to the nitride semiconductor layer. As a result, when operated in CW lasing up to a high output, they may suffer element breakdown without reaching a sufficient optical output. Thus, when they are fabricated as elements to be operated at an output as high as 100 mW or more, not only are the yields of properly functioning elements extremely low, but also, when actually operated for long durations, they may suffer sudden breakdown.